1. Field of the Invention
This invention relates generally to vacuum processing chambers which are widely used in the semiconductor industry, and in particular to chemical vapor deposition (CVD) processing chambers.
2. Description of the Related Art
The process of depositing layers on a semiconductor wafer (or substrate) usually involves heating the substrate and holding it a short distance from the source of a stream of deposition (or process) gas flowing towards the substrate. The flowing gas reacts and deposits a layer on the heated substrate.
A non-uniform gas flow pattern causes undesirable variations in the uniformity of the coating layer deposited across the surface of the wafer. Typically the exhaust port opening extending from the process chamber to the vacuum pump leads to asymmetry in the gas flow pattern. In an attempt to restore symmetry many designs provide a pumping channel which surrounds the wafer. However, in many configurations, symmetry is not possible because complete encirclement of the wafer is not possible due to interfering structures, e.g., the wafer insertion opening through which a robot arm passes the wafer into the chamber. Such chambers having mechanical asymmetry cannot achieve as high a gas flow symmetry as would be desired on the wafer.
The configuration of the channel and its lack of symmetry about the center of the wafer being processed affect the location at which the orifices can be placed and constrain the design based on the need to avoid other structures and passages in the chamber, e.g., the wafer insertion and removal opening and its path into and out of the chamber. In many instances the orifices to the pumping channel are configured in the chamber at locations which provide a less than ideal gas flow regime from the gas distribution plate to the pumping channel. Greater deposition takes place where the greatest number of reactants contact the substrate surface, i.e., around the vacuum apertures. The process gas flow resulting from such a configuration includes small but measurable variations in the thickness of the material deposited on the wafer.
At some point in the vacuum line between the chamber and the vacuum pump, a vacuum shut-off or throttle valve typically is provided which when closed acts as a limit of the process chamber containment. Volatile contaminants which are maintained in their vapor state under the high temperatures of an active processing chamber can and do undesirably condense on the walls of the cool vacuum piping at some distance from the processing area of the processing chamber, but still within the containment limit of the processing chamber defined by the vacuum valve. During no-flow conditions, these contaminants can migrate back into the processing part of the chamber to undesirably contribute to its contamination. In an attempt to avoid this problem, the inside surfaces within the containment limit of the processing chamber are periodically cleaned by etching (and/or plasma cleaning) these surfaces with a cleaning gas to remove the dielectric material deposited by the deposition gas. Optimization involves extending the time between cleanings as much as possible without introducing defects. Cleaning interferes with normal production processing.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.